A Gas Primer - Excessive Gas: Digestive Disorders

Content provided by the Faculty of the Harvard Medical School

A gas primer

The air we breathe is made up mostly of nitrogen (N₂) and oxygen (O₂), the gas the human body needs to sustain life. After being swallowed, air enters the GI tract. As it moves along, its makeup changes as oxygen passes into the blood and nitrogen is removed from the blood. Another intestinal gas is carbon dioxide (CO₂), a by-product of a chemical reaction with acid in the stomach. Hydrogen (H₂) is released in the colon when undigested carbohydrates undergo bacterial fermentation.

Bacteria in the gut produce foul-smelling gases when they ferment undigested foods that have not been absorbed in the small intestine. These foods consist mostly of carbohydrates, sugars, and fats. The carbohydrates found in high-fiber foods, such as beans, broccoli, cauliflower, and brussels sprouts, are the worst culprits. These foods release gases such as methane and hydrogen sulfate, which smells like rotten eggs. The worst odor is related to strong-smelling sulfurs that make up just 1% of flatus.

Methane is detected in about one-third of adults. Studies show that Americans and Europeans are more likely to produce methane than Asians are, possibly because of diet. Women also produce more than men do. Genes may play a role in methane production, as the trait is passed along in families.

Additional carbon dioxide is produced in the colon as the by-product of bacterial fermentation of unabsorbed sugars and starches. Eating beans will substantially increase CO₂ production, as will taking sodium bicarbonate for heartburn. Thus, it doesn't make sense to use bicarbonate-containing seltzers for gas.

Gas in the body

You hardly notice gas when it enters your digestive system, but once it's there, the discomfort can be intense. As for its escape, everyone knows the effect created by powerful gaseous emissions.

People who suffer the symptoms of gas pain and bloating usually have the same volume of gas as anyone else, or just a little bit more. But it affects them more because they are more sensitive. Some people with chronic gas pain have impaired peristalsis and significant reflux of gas from the small intestine into the stomach.

Although gas pain and bloating are usually not significant health concerns, it is important to see a doctor if the symptoms persist because they are occasionally a sign of some more serious condition. Severe distention immediately following a meal is called magenblase (stomach bubble) syndrome and may be mistaken for heart pain. Splenic flexure syndrome is a painful spasm in the left upper abdomen below the rib cage, produced by localized areas of trapped gas in the colon.

Borborygmi is an onomatopoeic word that refers to sounds created by peristaltic activity. Though disconcerting to the person whose insides are grumbling, it often goes unnoticed by anyone else.

Eating habits and gas

Throughout history, certain foods have been notorious for producing gas (see "Foods that may cause gas"). Beans are the most obvious example. Beans contain the complex carbohydrates stachyose and raffinose, which the intestine can't absorb but the bacteria in the colon love. The problem is most serious in people who have been eating a low-fiber diet and switch to a diet rich in beans and other high-fiber foods. Their digestive tracts don't have enough of the enzymes needed to digest bean sugars, which now pass undigested into the lower intestine where the bacteria metabolize them and generate gas. If people eat beans on a regular basis, the problem usually lessens as the body begins to produce the enzymes it needs.

People who are lactose intolerant often describe distressing flatulence if they consume milk products (see "Understanding food intolerance"). Other factors, such as disturbances in motility or metabolism, also influence how often and how much flatus is passed. For instance, people with slowed intestinal motility may produce more gas simply because bacteria have more time to work their magic on complex carbohydrates. Gas production may also increase when people take antibiotics, which lead to changes in the types of bacteria in the colon, or when the acidity level in the bowel goes down.

Last updated:	August 21, 2007